CLOS

1. CLOS CSC 358/458 6.3.2003

2. Outline • Homework #8 • CLOS • Basic features • OO programming • Examples

3. Homework #8 • Monkey • Student

4. CLOS • Adds object-oriented programming to Lisp • OOP is a choice • like C++

5. CLOS features • Built-in classes integrated with type system • User-defined classed also part of type system • Multiple inheritance • Class variables (static) and instance variables • Programmer-selected method combination • Multiple dispatch

6. defclass • creates a new class • supply • name • superclasses • slot specifiers • class options

7. Example (defclass submarine (boat) (speed depth))

8. make-instance • Creates an instance of a class • Example • (make-instance 'submarine)

9. slot-value • Allows access to defined slot • instance variables • Example • (setf s1 (make-instance 'submarine)) • (setf (slot-value s1 'speed) 100)

10. defmethod • Methods are defined separately from classes • Methods are specialized by the class they operate on

11. Example (defmethod dive ((sub submarine) new-depth) (setf (slot-value sub 'depth) new-depth))

12. Messaging Model of OOP • Most OO languages assume a message passing model • The method signature = message • Object with the method = recipient of message • But what about double-dispatch • transfer (account1, account2) • or even • equals (obj1, obj2) • We are forced to pick one object • implement method there

13. Dispatch Model

14. Dispatch Model of OOP • Methods independent of objects • Methods specialized for objects • but can be specialized for multiple objects

15. Example • In polynomials domain • add can be multiply defined • add (term poly) • add (term term) • add (poly term) • add (poly poly)

16. Slot specification • slot-value is awkward • can't be compiled • defclass has slot specifications • :accessor • :reader • :writer • :initform • :initarg • :allocation • :type

17. initform • Default initial value • Can be overridden by arguments to make-instance • Evaluated each time make-instance is called

18. initarg • A keyword that can be passed to make-instance • Gives slot its initial value • Overrides initform

19. Example (defclass Submarine () ((Depth :initarg :depth :initform 100) (Speed :reader Speed :initform 5))) (setq Sub-1 (make-instance 'Submarine :depth 200))

20. Accessors • reader • Name of a function that will read the slot • writer • Name of a function that will set the slot's value • accessor • Name of a function to read and (with setf) write the slot's value

21. Example 1 (defclass Submarine () ((Depth :reader Depth :initform 100) (Speed :reader Speed :initform 5))) (setq Sub-1 (make-instance 'Submarine)) (Depth Sub-1) ==> 100 (Speed Sub-1) ==> 5

22. Example 2 (defclass Submarine () ((Depth :reader Depth :writer Set-Depth :initform 100) (Speed :reader Speed :writer Set-Speed :initform 5))) (setq Sub-1 (make-instance 'Submarine)) (Depth Sub-1) ==> 100 (Speed Sub-1) ==> 5 (Set-Depth 200 Sub-1) (Set-Speed 4 Sub-1) (Depth Sub-1) ==> 200 (Speed Sub-1) ==> 4

23. Example 3 (defclass Submarine () ((Depth :accessor Depth :initform 100) (Speed :accessor Speed :initform 5))) (setq Sub-1 (make-instance 'Submarine)) (Depth Sub-1) ==> 100 (Speed Sub-1) ==> 5 (setf (Depth Sub-1) 200) (setf (Speed Sub-1) 4) (Depth Sub-1) ==> 200 (Speed Sub-1) ==> 4

24. type • Specifies the type of the argument • Can be used for optimization

25. allocation • Specifies instance or class allocation • Instance allocation means new copy of slot for each instance • instance variable in Java • Class allocation means one copy of instance for all instances • class variable in Java • declared with "static" (yuk!)

26. Other Class Options • documentation • doc string associated with class • default-initargs • useful for overriding initforms from superclass

27. defgeneric • Sort of like interface definition • Specify method name and argument list • Also • method-combination • argument-precedence-order

28. Method combination • How to inherit methods? • normally evaluate most-specific method • polymorphism! • But there are other possibilities • Discounts • Supplier discount = 10% • Preferred supplier discount = an additional 2% • We might want to specify that discounts are combined rather than overridden • in Java, must call super.method directly

29. In CLOS (defgeneric get-discount (firm) (:method-combination +)) (defclass supplier () ((discount :initform 10))) (defmethod get-discount + ((firm supplier)) (slot-value firm 'discount)) (defclass preferred-supplier (supplier) ()) (defmethod get-discount + ((firm preferred-supplier)) 2)

30. Method Combination • Specify type of method combination in generic function • Each method is marked with its combination method • Methods are called from least to most-specific • Combination methods • progn, +, list, and, or, max, min, append and nconc

31. Example • Polynomials • both terms and polynomials can be evaluated • We can change our code to create • term objects • polynomial objects

32. Method combination • What about canonicalization? • we'd like to have new instances canonicalized when they are created • Can do this with an "after" method • write a method that is applied after initialize-instance does its thing

33. Example

34. Another example • iterators • infinite iterators • integer iterators • list iterators • accumulation iterators • mapping iterators

35. Review • What have we covered in this class?